This invention relates to methods for manufacture of light guides with structured surfaces, and more particularly to continuous or semi-continuous methods for manufacturing light guides with structured surfaces.
Extrusion methods may be used to continuously manufacture an article using a thermoplastic resin feed. While extrusion processes are well adapted to melt process thermoplastic materials into articles with relatively smooth surface profiles, they are less suitable for creating articles having surface profiles with precision structures oriented in a direction transverse to a plane of a major surface of the article. In an extrusion process, the resin extrudate is still molten and soft as its exits the die, and the final surface profile of the article may change as the article cools. This profile change may be particularly troublesome if it is manifested as a volume change due to shrinkage as the resin cools, particularly if the volume change is large compared to the dimensions of the precision transverse structures on the major surface of the article.
Injection molding may also be used to manufacture articles having precision transverse surface profile structures using a thermoplastic resin feed. As the molten thermoplastic is injected into a mold cavity and contacts the raised or depressed structures oriented transverse to the longitudinal axis of the mold surface, the resin cools and forms a skin-like layer adjacent the mold surface. This skin-like layer prevents further flow at the mold surface, and the molten resin must be forced into the mold cavity at high pressure to fully fill the mold cavity. As a result, the contact between the thermoplastic and the transverse structures in the mold is not reliable, and it is difficult to replicate the structures with a high level of precision. In addition, since the injection molding process typically requires heating the resin feed to fill the mold, the molded article may shrink significantly when cooled.
For example, light transmitting articles such as light guides and light fibers often feature precise surface profile structures. These light transmitting articles are commonly manufactured using a thermoplastic extrusion or injection molding process. A light guide typically includes at least one surface adapted to receive light from a light source and an optically smooth surface for reflecting light propagating through the light guide. Common examples include optical fibers used in data communication and planar wave guides used in optical displays. A light fiber, commonly used in illumination systems, receives light in at least one end and transmits the light out of the fiber at a predetermined position or positions along the length of the fiber.
A typical light transmitting device is the light guide, which may be likened to a length of conventional optical fiber. Such a fiber generally has a core with a particular cross-sectional geometry (i.e., circular, elliptical, etc.) and a cladding on the core. The refractive index of the core is greater than the refractive index of the cladding. In use, a beam of electromagnetic energy, such as visible light, introduced into the core at one end of the fiber will always strike the core/cladding interface at an angle greater than the critical angle and so will be totally internally reflected. As a result, the light will be transmitted to the other end of the fiber without significant losses. See, for example, U.S. Pat. No. 5,898,810.
To extract light in a controlled manner from a light transmitting article, multiple light extraction structures may be formed in a direction transverse to a direction of light propagation in the article. The light extraction structures may be positive, projecting above a plane through which the light propagates in the light transmitting article, or negative, projecting below the plane of light propagation. Typically, the light transmitting article has an elongated shape, the light travels along a longitudinal axis of the article, and the light extraction structures are formed transverse to the longitudinal axis. At each structure, light is reflected at an angle that is less than the critical angle necessary for continued propagation along the article according to the principle of total internal reflection, which causes the light to exit the article.
The negative structures may be machined directly into the surface of the light transmitting article, and positive or negative structures may be imparted to the article in a batch molding process using a rigid or flexible mold. However, precision machining of the polymeric materials commonly used to make the light transmitting articles is time consuming, inaccurate and expensive. Likewise, other batch processing techniques may be difficult to conduct in a commercially feasible time period.
A continuous process for fabricating and then adhering a light-extracting structure to a light guide has been described in U.S. Pat. No. 6,039,553. In a first step, a continuous molding process for making a light transmitting article forms a molded layer with surface profile structures. In a second step, the molded layer is adhered to a light guide so that the structures are transverse to the direction of light propagation in the light guide. This continuous molding process more effectively replicates the structures needed to form a particular surface profile than conventional extrusion, injection molding and machining techniques, but the required two step molding procedure leaves an interface where the bonded layer is attached to the prefabricated light guide. Even if the bonded layer and the light guide are refractive index matched and aligned with great precision, the interface between the bonded layer and the light guide may interfere with the efficiency and precision of light transmission and/or extraction from the light transmitting article.
Currently, to impart precise structures into a surface of an article, the structures are machined into the surface of the article, the article is molded using a batch process, or the structures are applied to a surface of the article in a separate molding step. Conventional single step continuous extrusion and injection molding processes do not provide sufficiently accurate replication of surface profile structures transverse to a plane of a major surface of the article.
The invention is a continuous process for making an elongate light guide having surface profile structures arranged in a direction transverse to the longitudinal axis of the light guide. In the process of the invention, a liquid thermosettable material is introduced into a closed mold cavity having a molding surface with positive or negative structures oriented transversely to the longitudinal axis of the molding surface. During the molding process, as the thermosettable material undergoes polymerization to form a thermoset light guide, the structures in the mold are precisely imparted to a surface of the light guide in a single step.
In the process of the invention the liquid thermosettable material may be applied to the mold at a moderate temperature and a pressure selected to ensure that the liquid flows easily over or into the structures on the molding surface. The thermosettable material intimately and substantially uniformly contacts the structures in the mold, so that the transverse surface profile structures may be more precisely replicated than in conventional extrusion or injection molding processes. The moderate molding temperature decreases or substantially eliminates shrinkage of the fully or partially polymerized light guide. The process of the invention reduces cycle time and makes manufacture of the light guides more commercially feasible than conventional batch or multi-step molding processes.
The process of the invention is particularly well suited to the continuous, rapid manufacture of light transmitting articles such as light guides that include multiple light extraction structures in a direction transverse to a direction of light propagation in the light guide. The light extraction structures may be positive, projecting above a plane through which the light propagates in the light guide, or negative, projecting below the plane of light propagation. Typically, the light guide has an elongated shape, the light travels along a longitudinal axis of the light guide, and the light extraction structures are formed transverse to the longitudinal axis.
In one aspect, the invention is a method of fabricating an elongate light guide, including:
(a) providing a moving mold assembly comprising at least two mold parts, wherein the mold parts have an engaged portion and a non-engaged portion;
(b) moving the mold parts such that the non-engaged portions move in a first direction and the engaged portions move in a second direction different from the first direction to form an elongate regenerated mold cavity having a longitudinal axis, wherein the cavity comprises a molding surface with at least one structure transverse to the longitudinal axis;
(c) introducing a thermosettable material into the cavity;
(d) at least partially polymerizing the thermosettable material in the cavity to form a light guide therein; and,
(e) removing the light guide from the mold assembly.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.